This invention relates to an improved optical scanner which includes semiconductor charge coupled shift registers. More particularly, it relates to a charge coupled device that is so structured as to eliminate blooming.
Charge coupled devices store and transfer information in the form of electrical charge. They generally comprise metal electrodes on top of an insulating layer on top of a semiconductor substrate. In typical prior art devices, the semiconductor portion is, for the most part, homogeneously doped, regions of different conductivity being required only for injecting or extracting charge. (See, for example, Boyle et al, Bell System Technical Journal, 48,587 (1970).) In the shift register, a DC bias sufficient to deplete the semiconductor surface is applied between electrodes, and the semiconductor material and clocking pulses are applied sequentially to the electrodes. Because of the applied bias and clocking signals, semiconductor surface minority carriers are drawn to the semiconductor-insulator interface and tend to collect in the potential wells under the metal electrode. When the clocking pulses are sufficiently large, the minority carriers migrate from the area under one electrode to the area under the next following a potential well produced by the clocking pulses.
One major problem associated with the utilization of charge coupled device shift registers in optical scanning systems is the finite quantity of charge that can be stored in a potential well. When too much charge is generated within a potential well (for example, by localized intense light) the excess charge tends to spill over into adjacent potential wells. This phenomenon, known as "blooming", can cause a charge coupled optical scanner to react to a localized intense beam of light in the same manner that it would be expected to react to a more diffuse source of light, resulting in incorrect readings.
One prior art approach to the "blooming" problem utilizes an additional electrode that is adjacent to the electrodes which form the charge coupled shift register. The additional electrode is utilized as a reservoir to receive overflow electrical charge from the electrodes of the shift register. The reservoir action of the additional electrode is dependent upon having a portion of it overlay a reverse-biased semiconductor region to provide a sink for overflow charges which flow from the shift register electrodes. Among the disadvantages of this prior art approach are: the fact that an additional electrode is needed; its density disadvantage (because the silicon area required for such a structure is approximately doubled in size due to the lateral overflow reservoir); and its requirement of an additional level of metallization in, e.g., two dimensional arrays.
It is a general object of this invention to provide an improved charged coupled optical scanner.
A more particular object is the provision of a semiconductor charge optical scanner wherein "blooming" due to localized areas of intense light is prevented.
Another object is to accomplish the above without using additional electrodes.